# The Path to $0.01\%$ Theoretical Luminosity Precision for the FCC-ee

## TL;DR

This paper reviews the current theoretical precision of Bhabha luminometry and proposes pathways, including upgrades to Monte Carlo generators, to achieve the FCC-ee's target luminosity precision of 0.01%.

## Contribution

It identifies necessary improvements in Monte Carlo simulations and provides a detailed error budget analysis to meet FCC-ee luminosity precision goals.

## Key findings

- Upgrading BHLUMI can achieve 0.01% theoretical error.
- Analysis of Z and gamma exchanges informs precision improvements.
- Pathways outlined for Monte Carlo upgrades for FCC-ee requirements.

## Abstract

The current status of the theoretical precision for the Bhabha luminometry is critically reviewed and pathways are outlined to the requirement targeted by the FCC-ee precision studies. Various components of the pertinent error budget are discussed in detail -- starting from the context of the LEP experiments, through their current updates, up to prospects of their improvements for the sake of the FCC-ee. It is argued that with an appropriate upgrade of the Monte Carlo event generator BHLUMI and/or other similar MC programs calculating QED effects in the low angle Bhabha process, the total theoretical error of $0.01\%$ for the FCC-ee luminometry can be reached. A new study of the $Z$ and $s$-channel $\gamma$ exchanges within the angular range of the FCC-ee luminometer using the BHWIDE Monte Carlo was instrumental in obtaining the above result. Possible ways of BHLUMI upgrade are also discussed.